The use of laser scanning techniques for printing information on laser sensitive mediums have been disclosed in the prior art. For example, U.S. Pat. No. 3,922,485 discloses a multifaceted polygon optical scanner which scans a modulated laser beam across a xerographic medium. In order to print on the laser sensitive medium (i.e., the xerographic drum shown in the aforementioned patent), a laser of a particular output power is required. For example, the photoreceptor which comprises the xerographic medium disclosed in the aforementioned patent requires a laser flux of one milliwatt incident thereon to discharge predetermined charged areas of the photoreceptor to accomplish printing. In order to reduce the power requirements on the input laser which, in turn, would reduce its cost and size, the prior art has sought to optimize laser efficiency or in other words, the efficiency of the optical system such that maximum laser beam power is provided on the photoreceptor for a given input laser rated at a certain output power. One approach has been the optimization of the key components which comprise the optical system such as the modulator, polygon scanner and other major optical elements. However, the optical system reaches a certain point where efficiency does not increase. It has been found that typically optical scanning systems efficiencies are on the order of ten percent so that a ten milliwatt laser is required to apply one milliwatt of power on the photoreceptor. The impact of this performance is to require system designers to stress the laser power capability which in turn can effect the projected reliability, life, manufacturing cost, developemnt cost, and field operational costs. The end result of this projection may be to lessen the competitiveness of laser scanning systems of the type described in the aforementioned patent for printing applications.
It should be noted that the inefficiency of some of the components in the system is due to the contamination of various optical surfaces as well as glass-air interface light power losses. The surface losses of each optical element in the system effects the transmission of each element and cumulatively effects the efficiency of the overall scanning system. Further, in scanning systems which require more than one facet to be illuminated in order to reduce retrace times and provided a desired duty cycle such as that disclosed in the aforementioned patent, reduced system efficiencies are the result since only one beam from one facet can be utilized at a time. Generally, in order to provide a relatively uniform amount of light across the scan line, the beam illuminating the scanner facets is expanded to fully illuminate the facets. The end result of the beam expansion is that the percentage of light which can get through the scanner, even if the surfaces thereof were perfect reflectors, is severely reduced. The lower efficiencies inherent in illuminating two or more facets could be minimized by using a scanner facet dimension large compared to the optical beam incident at the polygon in the scan direction. Although this may be viable in a low resolution system or for a low speed scanner which can tolerate a large polygon dimension, this approach cannot be tolerated for high resolution systems or for high speed scanners.